Process for the production of N-(hydroxyphenyl) maleimides

ABSTRACT

N-(hydroxyphenyl) maleimides of the general formula ##STR1## where R&#39; stands for H, CH 3 , C 2  H 5 , F, Cl, Br or I and n is an integer of 1-5 are produced by treating the corresponding maleamic acid or by treating the ester of said N-(hydroxyphenyl) maleimide at a temperature of 0-150° C. in the presence of at least one dehydrating agent selected from the group consisting of oxides and oxyacids of sulfur or phosphorus and alkali metal and alkaline earth metal salts of the said oxyacids. The corresponding maleamic acid can be obtained by reacting an aminophenol having one or more hydroxyl groups on its phenyl nucleus with maleic anhydride. The esters of the N-(hydroxyphenyl maleimide) can be obtained by reacting said aminophenol and said maleic anhydride in the presence of a conventional acid anhydride dehydrating agent and a conventional imide-forming cyclization catalyst.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a novel process for the productionof N-(hydroxyphenyl) maleimides and, more particularly, to a process forproducing such maleimides by dehydrative cyclization ofN-(hydroxyphenyl) maleamic acids, obtained by the reaction ofaminophenols with maleic anhydride, with the use of a dehydrating agentselected from oxides and oxyacids of phosphorus or sulfur, and alkalimetal and alkaline earth metal salts of the oxyacids thereof, or byhydrolysis of esters of N-(hydroxyphenyl) maleimides in the presence ofat least one of said dehydrating agents. The ester may be obtained bythe reaction of aminophenols with maleic anhydride in the presence of aconventional imideforming catalyst and an acid anhydride serving as adehydrating agent.

2. Description of the Prior Art

For the production of maleimides, a process is generally known in whichan amine is reacted with maleic anhydride in the presence of a catalystsuch as triethylamine and sodium acetate using acetic anhydride as adehydrating agent. Such a process, however, is not applicable when theamine is an aminophenol having one or more hydroxyl groups on its phenylnucleus because the reaction inevitably involves esterification of thehydroxyl group due to its high reactivity, which takes place prior tothe cyclic condensation reaction, resulting in the failure to yield adesired N-(hydroxyphenyl) maleimide. Further, even with the use of acatalyst generally known to be effective, such as a metal salt includingsodium acetate, cobalt acetate, nickel acetate, manganese acetate,hydrates thereof and the like metal salt, and a base includingtriethylamine, N-dimethylbenzylamine, isoquinoline, triethylenediamine,pyridine, N-ethylmorpholine and the like base, in conjunction with aneffective dehydrating agent such as an acid anhydride includingpropionic anhydride, butyric anhydride, benzoic anhydride and the like,the hydroxyl group is first esterified with or without subsequent cycliccondensation so that the use of such catalysts and dehydrating agents isnot effective for the production of N-(hydroxyphenyl) maleimides.

To produce N-(hydroxyphenyl) maleimide derivatives, therefore, a methodhas been provided wherein the maleimide obtained in the abovementionedmanner and, thus, containing an ester group is subjected to atransesterification or hydrolysis treatment, such as disclosed inBelgian Pat. No. 613,801 in which N-(p-hydroxyphenyl) maleimide isproduced by transesterification of N-(p-acetoxyphenyl) maleimide.

However, the transesterification requires considerable time, e.g. over14 hours. In addition, it has been revealed from extensive examinationthat the yield of N-(p-hydroxyphenyl)-maleimide by transesterificationwith methanol for 21 hours is as low as 38%, and that the reactionrequires a large amount of methanol. Therefore, the conventional methodis not economical on an industrial scale.

SUMMARY OF THE INVENTION

It is, accordingly, an object of the present invention to provide aprocess for the production of N-(hydroxyphenyl)-maleimides, which isdevoid of the problems encountered in the conventional process.

The present inventors have found that high purityN-(hydroxyphenyl)-maleimide is obtained in both high yield and in shortreaction times by:

(a) hydrolyzing an ester of a N-(hydroxyphenyl) maleimide in thepresence of at least one dehydrating agent selected from the groupconsisting of oxyacids, of sulfur or phosphorus, and alkali metal saltsand alkaline earth metal salts of said oxyacids under mild temperatureconditions of 0°-150° C., or

(b) reacting an aminophenol with maleic anhydride with or without usinga solvent in the presence of a dehydrating agent selected from oxidesand oxyacids of sulfur or phosphorus, alkali metal salts and alkalineearth metal salts of said oxyacids, and mixtures thereof at atemperature of 0°-150° C. and for a period of 1-10 hrs to effect adehydrative cyclization reaction.

In accordance with the process of the present invention, there isprovided a process for producing a N-(hydroxyphenyl)-maleimide expressedby the general formula: ##STR2## where R' stands for H, CH₃, C₂ H₅, F,Cl, Br or I, and n is an integer of 1-5.

In a first aspect, this invention provides a process for producing themaleimide of formula (I), wherein an aminophenol of the general formula:##STR3## where R' and n each have the same meaning as indicated above,is reacted with maleic anhydride without using a solvent or in a polarsolvent such as dimethylformamide in the presence of a dehydrating agentwhich is an oxide or oxyacid of sulfur or phosphorus, an alkali metal oralkaline earth metal salt of said oxyacid, or mixtures thereof at atemperature of 0°-70° C., preferably 10°-40° C. for a period of 1-5hours to produce a N-(hydroxyphenyl) maleamic acid corresponding to themaleimide of formula I. The corresponding maleamic acid is of thegeneral formula: ##STR4## where R' and n each have the same meaning asdefined above. The reaction is further continued at a temperature of0°-150° C., preferably 50°-80° C. for a period of 1-5 hours to effectdehydrative cyclization of the maleamic acid.

In a second aspect, this invention provides a process for producing themaleimide of formula (I), in which the aminophenol of formula (II) isreacted with maleic anhydride without using a solvent or in a polarsolvent such as dimethylformamide at a temperature of 0°-70° C.,preferably 10°-40° C. for a time period of 1-5 hours to form theN-(hydroxyphenyl)-maleamic acid of formula (III). To the resultingreaction mixture is subsequently added a dehydrating agent which is anoxide or oxyacid of sulfur or phosphorus, an alkali metal or alkalineearth metal salt of the said oxyacid, or mixtures thereof. Thedehydrating agent may be added directly to the reaction mixture or afterdissolving it in the solvent. The reaction is further conducted at atemperature of 0°-150° C., preferably 50°-80° C. for a time period of1-5 hours thereby effecting dehydrative cyclization of the maleamicacid.

In a third aspect, this invention provides a process for producing themaleimide of formula (I), wherein an ester of a N-(hydroxyphenyl)maleimide expressed by the formula: ##STR5## where R' and n each has thesame meaning as defined hereinabove and R stands for an alkyl grouphaving 1 to 7 carbon atoms or a phenyl group, is subjected to hydrolysisat a temperature of 0°-150° C. in the presence of at least one of saiddehydrating agents, preferably selected from the group consisting ofoxyacids of sulfur or phosphorus. The ester may be obtained by reactingthe aminophenol of formula (II) with maleic anhydride in the presence ofa known imide-forming catalyst using an acid anhydride as a dehydratingagent.

According to the first and second aspects of the present invention,N-(hydroxyphenyl) maleimides are easily obtained in high yield withoutencountering any electrophilic attack to the reactive hydroxyl groupwhile minimizing the occurrence of undesirable, tar-forming sidereactions, in contrast with the conventional complicated processinvolving esterification and transesterification. Moreover, theN-(hydroxyphenyl) maleimides can be precipitated in crystalline form bymerely adding an inactive solvent such as water to the reaction solutionor by pouring the reaction solution into the inactive solvent. Thus, theseparation and purification of the product are greatly facilitated incomparison with those in the conventional process.

More specifically, according to the third aspect of the invention, theester compound of a N-(hydroxyphenyl) maleimide is first dissolved in anoxyacid of sulfur or phosphorus under relatively mild temperatureconditions, e.g. 0°-150° C., and the resulting solution is thengradually added to water to effect hydrolysis and to precipitate aN-(hydroxyphenyl) maleimide in crystalline form, the crystals beingseparated therefrom by conventional methods. This process ischaracterized by effecting the hydrolysis of the ester simultaneouslywith the addition of the oxyacid solution of the ester to water. In theconventional process, it is necessary to conduct a hydrolysis ortransesterification of the ester in the presence of a catalyst for along time to obtain the N-(hydroxyphenyl) maleimide. In contrast,according to the third aspect of this invention, the maleimide isobtained simply by the dissolution of the ester in the oxyacid followedby the addition of the resulting solution into ice water or cold water.In addition, the yield of the maleimide product is high and the productcan be easily separated and purified by any known way such ascentrifuging. Accordingly, the present invention provides an extremelyeconomical and industrially feasible process for the production ofN-(hydroxyphenyl) maleimides.

The presence of one or more highly reactive hydroxyl groups as well as adouble bond in the N-(hydroxyphenyl)-maleimides of the present inventionprovide the said imide with a variety of potential uses. For example,they can be used as raw materials for thermosetting resins obtained,e.g. by the reaction of the imide with epoxy resins, having excellentheat resistant properties and dimensional stability. They can also beused as raw materials for the production of photosensitive resins,impregnation varnishes, paints, copper clad laminates, strong adhesivesand the like.

DETAILED DESCRIPTION OF THE INVENTION

In one of the embodiments of the present invention, theN-(hydroxyphenyl) maleimide is produced by the dehydrative cyclizationof a N-(hydroxyphenyl) maleamic acid obtained by the reaction of anaminophenol with maleic anhydride.

The aminophenols which can be used for the process of the presentinvention are the compounds expressed by the formula: ##STR6## whereinR' stands for H, CH₃, C₂ H₅, F, Cl, Br or I and n is an integer of 1-5.Illustrative of the aminophenols are 2-methyl-p-aminophenol,2-chloro-p-aminophenol, p-aminophenol, m-aminophenol, o-aminophenol,4-aminocatechol, 5-amino-2-hydroxy-p-xylene, and3,5-dibromo-4-aminophenol.

The dehydrative cyclization of the maleamic acid is conducted in thepresence of a dehydrating agent, which may be added at the maleamicacid-forming stage or at the dehydrative cyclization stage.

The dehydrating agent used in the process of the present invention isselected from oxides and oxyacids of sulfur or phosphorus, and alkalimetal salts and alkaline earth metal salts of the oxyacids of sulfur andphosphorous. Illustrative of the dehydrating agents are diphosphorustrioxide, polymeric phosphorus dioxide, phosphorus pentoxide, phosphoricacid, metaphosphoric acid, pyrophosphoric acid, tripolyphosphoric acid,polyphosphoric acid, sulfur trioxide, disulfur trioxide, sulfuric acid,peroxydisulfuric acid, sodium sulfate, calcium sulfate, magnesiumsulfate, sodium phosphate, etc. These compounds may be used singly or incombination. Among these, polyphosphoric acid is preferable for reasonsof its strong dehydrating power, moderate chemical properties,capability of dissolving organic compounds and ease in handling. The useof sulfuric acid in conjunction with phosphorus pentoxide is alsopreferable because a synergistic effect is obtained due to thegeneration of sulfur trioxide. The dehydrating activity of the resultingmixture is superior to that attained when sulfuric acid and phosphorouspentoxide are used singly.

A variety of polar solvents may be used in the maleamic acid-formingand/or the subsequent dehydrative cyclization stages. Preferablesolvents include dimethylacetoamide, dimethylformamide,dimethylsulfoxide, N-methyl-2-pyrrolidone, dioxane, dimethyl sulfone andhexamethylphosphoramide. Dimethylformamide is most preferable since itmay form a chelate compound with phosphorus pentoxide or sulfur trioxidewhich serves to accelerate the dehydrative cyclization of the maleamicacids. The process of this invention, however, can be carried outwithout the abovementioned solvents when the dehydrating agent (e.g.sulfuric acid or polyphosphoric acid) also serves as a solvent.

In the process of this invention, maleic anhydride is generally used inan amount of 1-3 moles per mole of the aminophenol. However, since theN-(hydroxyphenyl) maleamic acid, the intermediate addition product, canbe obtained nearly quantitatively, it is sufficient to use 1-1.3 molesof maleic anhydride per mole of the aminophenol.

The amount of the dehydrating agent used is suitably 0.1 to 10 moles permole of the aminophenol or N-(hydroxyphenyl)-maleamic acid, whether thedehydrating agent is composed of a single component or of two or morecomponents.

The use of the dehydrating agent is effective even in the presence of aconventional catalyst for the dehydrative cyclization of maleamic acids,e.g. sodium acetate or triethylamine.

The reaction for forming the N-(hydroxyphenyl) maleamic acid from theaminophenol and maleic anhydride is generally performed at a temperatureof 0°-70° C., preferably 10°-40° C. At a temperature below 0° C., thereaction rate is low and much time is required for completing thereaction. At a temperature above 70° C., the double bond of theN-(hydroxyphenyl) maleamic acid product is thermally activated resultingin the formation of undesirable byproducts such as tarry substances.

The subsequent dehydrative cyclization of the N-(hydroxyphenyl) maleamicacid is performed generally at a temperature of 0°-150° C., preferably50°-80° C. At temperatures below 0° C., the reaction hardly proceeds,while, at temperature over 150° C., the highly reactive dehydratingagent tends to induce a number of side reactions so that tarrysubstances as well as various byproducts are produced. The desiredN-(hydroxyphenyl) maleimide product is considerably colored, andobtained in both lower purity and yield.

Reaction time is variable depending mainly upon the reactiontemperature. Generally, the N-(hydroxyphenyl) maleamic acid-formingreaction, at 10°-40° C., is conducted for 1-5 hours and the dehydrativecyclization of the maleamic acid is at 50°-80° C. for 1-5 hours.

In another embodiment of the present invention, the N-(hydroxyphenyl)maleimide is produced by the hydrolysis of esters of N-(hydroxyphenyl)maleimides expressed by the formula: ##STR7## wherein R, R' and n eachhas the same meaning as defined hereinabove. Illustrative of the estersare N-(4-acetoxyphenyl)-maleimide, N-(2-acetoxyphenyl) maleimide,N-(4-benzoyloxyphenyl)-maleimide, N-(4-butyryloxyphenyl) maleimide,N-(4-propionyl-oxyphenyl) maleimide, N-(4-acetoxy-2,6-dibromophenyl)maleimide, N-(4-acetoxy-3,6-dimethylphenyl) maleimide andN-(3,5-diacetoxyphenyl) maleimide.

The ester of formula (IV) may be a product obtained by the dehydrativecyclization of a N-(hydroxyphenyl) maleamic acid, which has beenproduced by the reaction of the aminophenol expressed by the formula(II) with maleic anhydride, in the presence of both: (a) a known,effective cyclization catalyst such as sodium acetate, cobalt acetate,nickel acetate, manganese acetate, hydrates thereof, triethylamine,N-dimethyl-benzylamine, isoquinoline, triethylenediamine, pyridine andN-ethylmorpholine, and (b) an acid anhydride, such as acetic anhydride,propionic anhydride, butyric anhydride and benzoic anhydride of theformula: ##STR8## where R stands for an alkyl group having 1 to 7 carbonatoms or a phenyl group. In this case, it is not necessary to isolatethe ester product from the resulting reaction mixture formed by theabove conventional reaction, i.e. the resulting reaction mixture as ismay be subjected to the subsequent hydrolysis according to the processof this invention to produce N-(hydroxyphenyl) maleimide.

The hydrolysis of the ester is preferably performed in the presence ofat least one oxyacid of sulfur or phosphorus. Illustrative of theoxyacids are sulfuric acid, thiosulfuric acid, sulfurous acid,phosphoric acid, metaphosphoric acid and pyrophosphoric acid. Theseoxyacids may be used singly or in combination. When the reactionsolution containing the ester product is treated with the dehydratingagent, maleamic acid which was not cyclized by the conventionalcyclization catalyst and acid anhydride is cyclized by the dehydratingagent.

It is preferred that the hydrolysis be carried out in such a manner thata solution containing the ester and the oxyacid is first prepared andthe solution is then added gradually to water.

Since the oxyacids of sulfur or phosphorus per se serve to act assolvents for the ester, the use of an additional solvent is unnecessaryto conduct the hydrolysis. However, nonreactive solvents such asdimethylformamide, dimethylacetamide, dimethylsulfoxide andN-methyl-2-pyrrolidone may be suitably employed to enhance thedissolution of the ester.

The concentration of the ester in the solution to be added to water iswithin the solubility limit thereof, preferably, in the range of 80% to90% of the solubility limit depending on the stability of the solutionand the time required to completely dissolve the ester in the solventsystem consisting of the oxyacid alone or the oxyacid and theabove-mentioned additional solvent.

The dissolution of the ester in the solvent system is performed at atemperature of 0°-150° C., preferably 20°-80° C. Operation at atemperature below 0° C. is disadvantageous economically due to the lowsolubility of the ester in the solvent system. On the other hand, at atemperature above 150° C., undesirable side reactions occur between theester and the oxyacid, resulting in the lowering in the yield of thedesired product. Further, when the solution is added to water, thetemperature of the water is considerably raised, sometimes to over itsboiling point, due to the great heat of dilution of the solution,inducing various further side reactions. Thus, the yield of the desiredN-(hydroxyphenyl) maleimide is lowered and the separation andpurification of the end product become extremely difficult andtroublesome.

The time required to dissolve the ester in the solvent system dependsupon the type and amount of the ester, the dissolution temperature andthe rate of stirring. Generally, a period of 0.5 to 5 hours issufficient to obtain a solution with an ester concentration of 80%-90%of the solubility limit thereof.

The temperature of the water to which the ester solution is added toeffect the hydrolysis, is preferably 0°-30° C. within which range therise of the temperature of the reaction solution due to the heat ofdilution is suitably suppressed. It is possible to use ice water or iceto conduct the hydrolysis.

N-(hydroxyphenyl) maleimides are partially soluble in water. In theproduction of such imides, it is preferred that the water containcertain salts with salting-out capabilities, such as sodium chloride,sodium sulfate and calcium chloride, in order to improve theprecipitation efficiency of the product. Alternatively, the imideproducts dissolved in the aqueous reaction solution may be suitablyrecovered by extraction with ether, benzene, toluene, xylene or thelike.

The following examples further illustrate the process of the invention.

EXAMPLE 1

50 g (0.216 moles) of N-(4-acetoxyphenyl) maleimide were dissolved in100 g of sulfuric acid at 50° C. with stirring. It took 45 minutes tocompletely dissolve the esterified imide. The resulting solution wasthen gradually added to ice water which was a mixture of 200 g of iceand 100 ml of water, thereby precipitating N-(4-hydroxyphenyl) maleimideas crystals. The crystals were separated by filtration and dried toobtain 25.8 g (0.136 moles) of N-(4-hydroxyphenyl) maleimide (yield:63.0%).

    ______________________________________                                        Elementary analysis                                                           ______________________________________                                        Found       C:     63.33   H:   3.80  N:   7.34                               Calculated  C:     63.49   H:   3.73  N:   7.40                               ______________________________________                                    

The results of the elementary analysis and infrared absorption spectrarevealed that the acetoxy group is converted to a hydroxyl group.

EXAMPLE 2

49.0 g (0.200 moles) of N-(4-propionyloxyphenyl) maleimide weredissolved in 100 g of phosphoric acid at 60° C. It took 60 minutes tocompletely dissolve the esterified imide. The solution thus obtained wasthen added gradually to ice water of 0° C. to precipitateN-(4-hydroxyphenyl) maleimide. After filtration and drying, 22.7 g(0.120 moles) of yellowish orange crystals were obtained. The meltingpoint of the crystals was found to be 182°-183° C. An infraredabsorption spectra of the product revealed the presence of a hydroxylgroup, and the product was identified as N-(4-hydroxyphenyl) maleimide.

The yield was 60.1%.

EXAMPLE 3

In a reactor, 50 ml of dimethylformamide and 21.6 g (0.22 moles) ofmaleic anhydride were placed, to which were gradually added 21.8 g (0.20moles) of p-aminophenol at 20° C. with thorough stirring. Aftercompletion of the addition, the reaction was allowed to proceed at 20°C. for 2 hours to yield N-(p-hydroxyphenyl) maleamic acid. To thereaction mixture, a previously formed solution containing 70 ml ofdimethylformamide, 11.4 g (0.0803 moles) of phosphorus pentoxide and 5 g(0.049 moles) of sulfuric acid, was added dropwise with the use of adropping funnel. Thereafter, the reaction was continued at 70° C. for 2hours. After cooling to room temperature, the resulting reaction mixturewas poured into 400 ml of ice water to precipitate N-(p-hydroxyphenyl)maleimide in the form of crystals. The crystals were separated byfiltration, washed with water and then dried to obtain 31.3 g (0.165moles) of N-(p-hydroxyphenyl) maleimide (yield 82.5%).

EXAMPLE 4

Using 2-amino-5-hydroxytoluene as a starting aminophenol derivative andsulfur trioxide as a dehydrating agent, N-(2-methyl-4-hydroxyphenyl)maleimide was prepared in the same manner as in Example 3. The amount ofmaterials used were as indicated in Table 1. The results are also shownin Table 1.

EXAMPLE 5

Using 4-aminocatechol as a starting aminophenol derivative andpolyphosphoric acid and sulfuric acid as a dehydrating agent,N-(3,4-dihydroxyphenyl) maleimide was prepared in the same manner as inExample 3. The amount of materials used together with the results wereas indicated in Table 1.

                  TABLE 1                                                         ______________________________________                                                      Example 4                                                                             Example 5                                               ______________________________________                                        2-Amino-5-hydroxytoluene                                                                     36.9 g     --                                                                 (0.30 moles)                                                   4-Aminocatechol                                                                              --         12.5 g                                                                        (0.10 moles)                                        Maleic anhydride                                                                             32.4 g     11.7 g                                                             (0.33 moles)                                                                             (0.12 moles)                                        Dimethylformamide                                                                            80 ml      30 ml                                               Sulfur trioxide                                                                              52 g       --                                                                 (0.65 moles)                                                   Polyphosphoric acid                                                                          --         23 g                                                                          (P.sub.2 O.sub.3 =0.136 moles)                      Sulfuric acid  --         5 g                                                                           (0.049 moles)                                       Maleimide produced                                                                           42.1 g     14.8 g                                              yield          (69.2%)    (72.4%)                                             ______________________________________                                    

EXAMPLE 6

In a reactor, 30.0 g (0.275 moles) of p-aminophenol, 29.7 g (0.303moles) of maleic anhydride, 39.0 g (0.275 moles) of phosphorus pentoxideand 100 g of dimethylformamide were charged and stirred at 15° C. for 2hours. The reaction mixture was then gradually heated to 60° C., atwhich temperature the reaction was further continued for 2.5 hours.After cooling to room temperature, the resulting reaction mixture wasadmixed with water to precipitate N-(p-hydroxyphenyl) maleimide in theform of crystals. The crystals were separated by filtration, washed withwater and then dried to obtain 25.1 g (0.133 moles) ofN-(p-hydroxyphenyl) maleimide (yield 48.2%).

EXAMPLE 7

In a reactor, 40 ml of N-methyl-2-pyrrolidone and 7.1 g (0.05 moles) ofphosphorus pentoxide were charged, to which were added gradually 20.7 g(0.10 moles) of N-(m-hydroxyphenyl) maleamic acid at 20° C. withsufficient stirring. Thereafter, the reaction was continued at 70° C.for 3 hours. After cooling to room temperature, the resulting reactionmixture was admixed with water to precipitate N-(m-hydroxyphenyl)maleimide in the form of crystals. The crystals were separated byfiltration, washed with water and then dried to obtain 14.8 g (0.0782moles) of N-(m-hydroxyphenyl) maleimide (yield 78.2%).

EXAMPLE 8

In a reactor, 70 ml of dimethylformamide, 8.3 g (0.058 moles) ofphosphorus pentoxide and 7.1 g (0.05 moles) of sodium sulfate werecharged, to which 36.5 g (0.10 mole) of N-(3,5-dibromo-4-hydroxyphenyl)maleamic acid were gradually added at 25° C. with vigorous stirring.Thereafter, the reaction was continued at 60° C. for 3 hours. Aftercooling to room temperature, the resulting reaction mixture was addedwith water to precipitate N-(3,5-dibromo-4-hydroxyphenyl) maleimide inthe form of crystals. The crystals were then separated by filtration,washed with water and dried to obtain 26.3 g (0.0757 moles) ofN-(3,5-dibromo-4-hydroxyphenyl) maleimide (yield 75.7%).

EXAMPLE 9

N-(p-hydroxyphenyl) maleamic acid is obtained as in Example 3. Themaleamic acid is cyclically condensed in the presence of sodium acetateand acetic anhydride to obtain a solution containing N-(4-acetoxyphenyl)maleimide. The solution is admixed with sulfuric acid at 50° C. todissolve the esterified imide. The resulting solution is added to icewater and the precipitated crystals of N-(4-hydroxyphenyl) maleimide areseparated by filtration and dried as described in Example 1.

EXAMPLE 10

The procedure of Example 9 is repeated with the exception that theN-(4-acetoxyphenyl) maleimide is isolated from the reaction solution byfiltration and the isolated maleimide is admixed with the sulfuric acid.

What is claimed is:
 1. A process for the production of aN-(hydroxylphenyl)-maleimide of the general formula: ##STR9## where R'stands for H, CH₃, C₂ H₅, F, Cl, Br or I and n is an integer of 1-5,comprising treating a N-(hydroxyphenyl)-maleamic acid expressed by thegeneral formula: ##STR10## where R' and n each have the same meaning asindicated above, at a temperature of 0°-150° C. with a dehydrating agentselected from the group consisting of oxides and oxyacids of sulfur orphosphorus, alkali metal and alkaline earth metal salts of said oxyacidsand mixtures thereof to effect dehydrative cyclization of the maleamicacid, said maleamic acid being obtained by the reaction of anaminophenol expressed by the general formula: ##STR11## where R' and neach have the same meaning as indicated above, with maleic anhydride. 2.A process as claimed in claim 1 wherein said dehydrating agent is anoxyacid of sulfur or phosphorus.
 3. A process as claimed in claim 1wherein an inactive solvent is admixed with the reaction solution toprecipitate the N-(hydroxyphenyl) maleimide.
 4. A process as claimed inclaim 3 wherein the inactive solvent is water.
 5. A process as claimedin claim 1 wherein said aminophenol is a p-aminophenol.
 6. A process asclaimed in claim 1 wherein said dehydrating agent is polyphosphoricacid.
 7. A process as claimed in claim 1 wherein said dehydrating agentis a mixture of sulfuric acid and phosphorus pentoxide.
 8. A process asclaimed in claim 1 wherein the dehydrative cyclization is performed in apolar solvent.
 9. A process as claimed in claim 8 wherein said polarsolvent is dimethylformamide.
 10. A process as claimed in claim 1wherein the dehydrative cyclization is performed without using asolvent.
 11. A process as claimed in claim 1 wherein the aminophenol offormula III is reacted with the maleic anhydride in the presence of saiddehydrating agent at a temperature of 0°-70° C. to form a mixturecontaining said maleamic acid of formula II, and said mixture is furthertreated to effect the dehydrative cyclization.
 12. A process as claimedin claim 1 wherein the aminophenol of formula III is reacted with themaleic anhydride at a temperature of 0°-70° C. to form a mixturecontaining said maleamic acid of formula II, and said dehydrating agentis subsequently added to said mixture to effect the dehydrativecyclization.